1. Field of the Invention
The present invention relates to a semiconductor device for use in a power converter such as an inverter or the like, and a method of designing the semiconductor device.
2. Description of Related Art
An insulated gate type semiconductor device is used for a power converter such as an inverter for large current or the like, and IGBT (Insulated Gate Bipolar Transistor) is typically known as an element for the insulated gate type semiconductor device, for example. Furthermore, a semiconductor device having plural IGBT cells which are provided in a stripe-like arrangement or a mesh-like arrangement to reduce ON voltage is also known. An emitter electrode common to some IGBT cells is provided on the surface of the thus-constructed semiconductor device, and plural bonding wires extending from peripheral electrodes provided on the periphery of the semiconductor device are bonded and electrically connected to the emitter electrode at suitable intervals.
Furthermore, it is known that when latch-up occurs in such a semiconductor device, large current flows through each bonding wire, and the current density just below the bonding wire becomes larger than that at the circumference of the bonding wire, so that the IGBT cells just below the bonding wires are broken.
A technique of suppressing the current density just below bonding wires by improving the imbalance of a current distribution in the semiconductor device is disclosed in JP-A-10-112541, for example.
According to the disclosure of this technique, it is indicated that the wire resistance between the emitter electrode of an IGBT and a bonding wire is lower as the IGBT cell concerned is nearer to the bonding wire. In other words, larger current loss occurs between the emitter electrode of an IGBT cell and the bonding wire due to relatively large wire resistance as the IGBT cell concerned is farther away from the bonding wire, however, current loss is small in an IGBT cell just below the bonding wire. As a result, it is suggested that a nonuniform current distribution having a peak just below the bonding wire is obtained.
Furthermore, according to the technique described above, current loads of the respective IGBT cells which are different in accordance with the distance from the bonding wire are made uniform so that the current distribution uniform is uniform. In order to make the current load uniform among the IGBT cells, the IGBT cells are designed so that the size of the IGBT cell is reduced to increase the current density at a place which is far away from the bonding wire and at which the current load is small. Conversely, the size of the IGBT cell is increased to lower the current density at a place which is near to the bonding wire and at which the current load is large. Accordingly, the current distribution is made uniform in the semiconductor device, and the concentration of current just below the bonding wires can be suppressed.
However, the difference in wire resistance between the IGBT cell at the place far away from the bonding wire and the IGBT cell at the place near to the bonding wire is not so large that it does not remarkably affect the difference between current loses of these IGBT cells. Therefore, the current distribution cannot be sufficiently made uniform by varying the size of the IGBT cell in accordance with the distance from the bonding wire, and thus the current density just below the bonding wire cannot be suppressed. In addition, when the size of the IGBT cell is varied in accordance with the distance from the bonding wire, the circuit design and manufacturing process are cumbersome.